Conventionally, there has been conducted a research regarding a high electron mobility transistor (HEMT), in which an AlGaN layer and a GaN layer are formed over a substrate through crystal growth, and the GaN layer functions as an electron transit layer. A band gap of GaN is 3.4 eV, which is larger than a band gap of GaAs (1.4 eV). For this reason, a GaN-based HEMT has a high withstand voltage, and it is promising as a high withstand voltage electronic device for automobiles and so on.
Further, as structures of the GaN-based HEMT, there are a horizontal structure, in which a source and a drain are disposed in parallel to a surface of a substrate, and a vertical structure, in which a source and a drain are disposed perpendicular to a surface of a substrate.
In the vertical structure, since a current path becomes three-dimensional, it is possible to increase a current amount per chip compared to the horizontal structure. Further, a drain electrode and a source electrode are positioned at top and bottom of the substrate, so that even when areas of these electrodes are enlarged, it is easy to reduce an area of the chip. Therefore, even when the areas of the drain electrode and the source electrode are enlarged for letting a large current flow, the area of the chip hardly increases. In addition, since a ratio of metal per chip is increased, a heat release characteristic is improved.
In the GaN-based HEMT, even when a voltage is not applied to a gate, a current flows in a channel because two-dimensional electron gas resulting from a difference in lattice constants between a GaN layer and an AlGaN layer exists. Specifically, a normally-on operation is performed. Meanwhile, it is also conceivable that a current flows between the source and the drain when a power is turned on, when the gate electrode is broken or the like, in which 0 V is unintentionally applied to the gate electrode. Accordingly, a normally-off operation of the GaN-based HEMT is desired also from a point of view of fail-safe.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-140368    Non-Patent Document 1: Japanese Journal of Applied Physics vol. 46, No. 21, 2007, pp. L503 to L505